The goal is to get the planes flying passengers again, according to multiple sources with knowledge of the matter, while Boeing works on a comprehensive redesign of the lithium-ion battery system that could take nine months or more to implement.

The interim fix includes a heavy-duty titanium or steel containment box around the battery cells, and high-pressure evacuation tubes that, in the event of a battery fire, would vent any gases directly to the outside of the jet.

Boeing's approach implicitly acknowledges that four weeks after two batteries overheated — one catching fire on the ground, the other smoldering in flight — investigators have still not pinpointed the cause.

That leaves Boeing little choice for now but to engineer a solution that will better contain any such incident and protect the airplanes.

But it's unclear if the Federal Aviation Administration is ready to accept containment of an overheated battery cell rather than prevention.

"We're not there yet," said a government official with knowledge of the ongoing discussions, who asked not to be named by the Seattle Times. "It wouldn't surprise me if we're still talking weeks before everyone is comfortable."

Even if the FAA agrees, the short-term fix will take at least three months to design, test, certify and retrofit, said an Everett, Wash., source who knows details of Boeing's proposed solutions.

That would mean the earliest the Dreamliners could fly passengers again would be May. If it's much longer than that, assembly of the jets in Everett will probably have to be slowed and Boeing's plan to ramp up production will be severely disrupted, he said.

"This cannot drag out for six to nine months — from a financial standpoint. Think about nine months of airplanes just sitting there," said the Everett source. "This is a gut-wrenching issue."

Boeing will not disclose any details of the solutions it is working on.

But unlike Airbus, which last week said it will switch to nickel-cadmium main batteries for its forthcoming A350 jet to avoid the possibility of delays, Boeing insists it will stick with the high-energy lithium-ion batteries that provide emergency backup power for the 787.

"Boeing is confident in the safety and reliability of lithium-ion batteries," said spokesman Marc Birtel, and "good progress is being made" in resolving the battery problem.

But aviation experts are increasingly worried.

Ken Herbert, senior vice president with Los Angeles-based investment bank Imperial Capital, wrote in a note to investors Friday that "there are still considerable concerns as to whether the FAA will sign off on a solution that contains a potential battery fire, rather than one that prevents a fire."

"The risk to Boeing and the supply chain as a result of the 787 grounding is increasing," Herbert wrote. "We believe the grounding costs Boeing over $25 million a month in direct costs, and the total cost to Boeing could be over $1 billion."

That will add to the 787's one-time development costs, which financial analysts estimate have already cost Boeing somewhere between $15 billion and $20 billion.